This post doesn't really tell us what Udacity does with data (we can only imagine what it does with data, and I can't imagine it's entirely savory) but it does have some nifty diagrams. I especially like the user activity flow diagram (depicted along with this post). The trick with data is to get beyond the obvious categorizations ('is active', 'is dormant') to get to nuanced distinctions that predict genuine differences in behaviour. But this is likely always to be stochastic, which limits its applicability in the individual case.